Method of semiconductor nanocrystal synthesis

ABSTRACT

A method is described for the manufacture of semiconductor nanoparticles. Improved yields are obtained by use of a reducing agent or oxygen reaction promoter.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority under 35 U.S.C. § 119(e)(1) to U.S.Provisional Application Ser. No. 60/326,746, filed Oct. 2, 2001; U.S.Provisional Application Ser. No. 60/401,671, filed Aug. 6, 2002; andU.S. Provisional Application Ser. No. 60/404,628, filed Aug. 19, 2002;the disclosures of which are incorporated by reference herein in theirentireties.

FIELD OF THE INVENTION

This invention relates to nanoparticles. More particularly, theinvention relates to methods for making and using semiconductornanoparticles. The invention finds utility in a variety of fields,including biology, analytical and combinatorial chemistry, medicaldiagnostics, and genetic analysis.

BACKGROUND OF THE INVENTION

Semiconductor nanocrystals (also known as quantum dot particles) whoseradii are smaller than the bulk exciton Bohr radius constitute a classof materials intermediate between molecular and bulk forms of matter.Quantum confinement of both the electron and hole in all threedimensions leads to an increase in the effective band gap of thematerial with decreasing crystallite size. Consequently, both theoptical absorption and emission of semiconductor nanocrystals shift tothe blue (higher energies) as the size of the nanocrystals gets smaller.

Semiconductor nanocrystals are nanoparticles composed of an inorganic,crystalline semiconductive material and have unique photophysical,photochemical and nonlinear optical properties arising from quantum sizeeffects, and have therefore attracted a great deal of attention fortheir potential applicability in a variety of contexts, e.g., asdetectable labels in biological applications, and as useful materials inthe areas of photocatalysis, charge transfer devices, and analyticalchemistry. As a result of the increasing interest in semiconductornanocrystals, there is now a fairly substantial body of literaturepertaining to methods for manufacturing such nanocrystals.

In general, these routes can be classified as involving preparation inglasses (Ekimov et al., JETP Letters 34:345 (1981)); aqueouspreparation, including preparations that involve use of inversemicelles, zeolites, Langmuir-Blodgett films, and chelating polymers(Fendler et al., J. Chem. Society, Chemical Communications 90:90 (1984)and Henglein et al., Ber. Bunsenges. Phys. Chem. 88:969 (1984)); andhigh temperature pyrolysis of organometallic semiconductor precursormaterials, i.e., rapid injection of precursors into a hot coordinatingsolvent (Murray et al., J. Am. Chem. Soc. 115:8706 (1993) and Katari etal., J. Phys. Chem. 98:4109 (1994)). The two former methods yieldparticles that have unacceptably low quantum yields for mostapplications, a high degree of polydispersity, poor colloidal stability,a high degree of internal defects, and poorly passivated surface trapsites. In addition, nanocrystals made by the first route are physicallyconfined to a glass matrix and cannot be further processed aftersynthesis.

Improved synthesis conditions have been reported that utilize cadmiumsalts (Peng, et al., J. Am. Chem. Soc. 123:183-184 (2001)). Theseconditions provide certain advantages over the rapid injection method.The use of cadmium acetate, cadmium oxide or other such Cd(II) salts,pre-complexed with a ligand such as tetradecylphosphonic acid providesfor a cadmium precursor that is particularly suitable for nanocrystalsynthesis. These reactions have numerous desirable features, includingimproved safety and relatively wide tolerance for production variablessuch a precursor injection rate and temperature. Of particular note isthat these reactions can be tuned to yield very narrow photoluminescencespectra over a wide range of useful wavelengths. Unfortunately, it isdifficult to optimize the particle yield, while maintaining thedesirable features of the Cd(II) synthesis conditions. In particular,for smaller size nanoparticle synthesis, yields have been very poorunder Cd(II) synthesis conditions. Reaction conditions that provide suchlow yields are not only more expensive to implement on a manufacturingscale, but they often require much larger reactors and produce morehazardous waste.

Thus, there remains a need in the art for improved methods formanufacturing nanoparticles, and smaller nanoparticles in particular.Such methods would ideally provide a high product yield of internallydefect free, high band edge luminescence nanoparticles with no orminimal trapped emission. Such methods would also ideally provide forthe manufacture of particles that exhibit near monodispersity and have arelatively narrow particle size distribution. Finally, such methodswould be useful not only with semiconductor nanoparticles, but also withother types of nanoparticles, e.g., semiconductive nanoparticles thatare not necessarily crystalline and metallic nanoparticles.

The present invention addresses those needs by providing improvedmethods for manufacturing nanoparticles. By controlling the nucleationdensity the methods of the invention provide for a predictable andcontrollable final particle size, as well as many of the aforementionedproperties.

SUMMARY OF THE INVENTION

One aspect of the invention relates to a method of producingnanoparticles comprising: (a) mixing a first precursor and at least onecoordinating solvent to form a first mixture; (b) exposing the firstmixture to a reaction promoter selected from the group consisting ofoxygen and a reducing agent; (c) heating the first mixture to atemperature that is sufficiently high to form nanoparticles (nucleationcrystals) when a second precursor is added; (d) introducing a secondprecursor into the first mixture to form a second mixture therebyresulting in the formation of a plurality of nanoparticles; and (e)cooling the second mixture to stop further growth of the nanoparticles.

Another embodiment of the invention is a method of producingnanoparticles comprising: (a) mixing a first precursor and a secondprecursor with at least one coordinating solvent to form a firstmixture; (b) heating the first mixture to a temperature that issufficiently high to form nanoparticles when a reaction promoter isadded; (c) exposing the first mixture to a reaction promoter, thereaction promoter being selected from the group consisting of oxygen anda reducing agent, to form a second mixture thereby resulting in theformation of a plurality of nanoparticles; and (d) cooling the secondmixture to stop further growth of the nanoparticles.

Yet another embodiment of the invention is a method of producing thenanoparticle shell comprising (a) mixing nanoparticles with at least onecoordinating solvent to form a first mixture; (b) heating the firstmixture to a temperature that is sufficiently high to form a shell onthe nanoparticles when first and second precursors are added; (c)introducing first and second precursors into the first mixture to form asecond mixture thereby resulting in the formation of shells on aplurality of nanoparticles; and (c) cooling the second mixture to stopfurther growth of the shell; wherein the method further comprisesexposing the first or second mixture to a reaction promoter selectedfrom the group consisting of oxygen and a reducing agent. Thenanoparticles can be produced by the methods described herein or can beproduced by any method known in the art.

Another aspect of the invention pertains to a method of producingsemiconductive nanoparticles having a valency “n”, comprising: (a)mixing a first precursor having a valency “c” and at least onecoordinating solvent to form a mixture; (b) exposing the mixture to areaction promoter wherein the reaction promoter converts the valency ofthe first precursor to a valency “a”; (c) heating the mixture to atemperature that is sufficiently high to form nanoparticles when asecond precursor is added; (d) introducing a second precursors into themixture to form a second mixture thereby resulting in the formation of aplurality of nanoparticles; wherein the second precursor has a valency“b”, and wherein a+b=n and c+b≢n; and (e) cooling the second mixture tostop further growth of the nanoparticles.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 and FIG. 2 are graphical representations of the effect ontemporal wavelength evolution during the course of a CdSe nanocrystalcore-forming reaction as a function of added reaction promoter, asdescribed in Examples 1 and 2. FIG. 1 shows the effect on emission peakwavelength, while FIG. 2 shows the effect on the full peak width at halfmaximum (FWHM).

FIG. 3 and FIG. 4 are graphical representations of the effect ofincreasing amounts of diphenylphosphine (DPP) on emission peakwavelength and FWHM as described in Example 1.

FIG. 5 is a graphical representation of the particle counts recoveredfrom each of the reactions shown in the FIG. 2 and illustrates the levelof control afforded by the use of reaction promoters.

FIG. 6 is a graphical representation of the effect of the time of airexposure prior to the TOPSe injection as described in Example 3. Thescale on the left-hand coordinate is the emission peak wavelength innanometers and the right-hand ordinate is the FWHM peak height innanometers.

FIG. 7 is a graphical representation of the effect of a spike ofair-exposed TOP before the TOPSe injection as described in Example 5.

FIG. 8 is an illustration of one implementation of an electrochemicalsystem which could be used to prepare nanocrystals by the methodsdescribed here.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides an improved method of producingnanoparticles having a means for controlling the reactivity of asolution of nanoparticle precursors through introduction of a reactionpromoter prior to the nucleation of nanoparticles. This control ofreactivity allows for control of the relative number of nanoparticlenuclei formed in a single nucleation period. Control of nucleationdensity provides for a predictable and controllable final nanoparticlesize, size distribution and yield. In addition, the method also providesfor control of the growth rate once nucleation occurs, which allows forcontrol of size focusing, resulting in very narrow size distributions.The methods described herein can be used to form the nanoparticle core,the nanoparticle shell, or both. In addition, the methods describedherein can be used to form nanoparticles that are smaller than has beenpossible using traditional methods known in the art, e.g., for preparingCdSe nanoparticles which emit with an emission maximum in the range of400 nm to 500 nm.

In general, the invention provides a method of producing nanoparticlesby contacting a first precursor M′ (valency=c) with a reaction promoter,wherein the reaction promoter converts M′ to M (valency=a); and thencontacting the M precursor with a second precursor X (valency=b) toproduce nanoparticles having a valency “n”, wherein c+b≢n and a+b=n.

Either the first elemental component or the second elemental componentof the nanoparticle can gain electrons. Therefore, the resultingnanoparticle can have a composition of MX or XM, as exemplified below.For example, the method of the invention can be used for the productionof CdSe nanoparticles, i.e., MX nanoparticles where n=0. The firstprecursor M′ can be Cd⁺²(c=⁺2). Upon reaction with a reaction promoterof the invention, such as DPP or hydroquinone, Cd⁺² is converted orreduced to Cd⁰ (M, where a=0). This is then contacted with the secondprecursor Se⁰ (X, where b=0) to produce the CdSe nanoparticles having avalency of 0.Cd⁺²+reaction promoter→Cd⁰ Cd⁰+Se⁰→CdSeThe method of the invention can also be used for the production of, forexample, InP nanoparticles, i.e., XM nanoparticles where n=0. The firstprecursor M′ can be P⁰ (c=0). Upon reaction with a reaction promoter ofthe invention, such as DPP or hydroquinone, P⁰ is converted or reducedto P⁻³ (M, where a=⁻3). This is then contacted with the second precursorIn⁺³ (X, where b=⁺3) to produce the InP nanoparticles having a valencyof 0.P⁰+reaction promoter→P⁻³ In⁺³+P⁻³→InP

Accordingly, one embodiment of the invention is a method of producingsemiconductive nanoparticles having a valency “n”, comprising: (a)mixing a fist precursor having a valency “c” and at least onecoordinating solvent to form a first mixture; (b) exposing the firstmixture to a reaction promoter wherein the reaction promoter convertsthe valency of the first precursor to a valency “a”; (c) heating thefirst mixture to a temperature that is sufficiently high to formnanoparticles when a second precursor is added; (d) introducing a secondprecursor into the mixture to form a second mixture thereby resulting inthe formation of a plurality of nanoparticles; wherein the secondprecursor has a valency “b”, and wherein a+b=n and c+b≢n; and (e)cooling the second mixture to stop further growth of the nanoparticles.

More specifically, in one embodiment of the invention, the method ofproducing nanoparticles is a one-pot synthesis technique that involves:(a) mixing a first precursor, an optional ligand and at least onecoordinating solvent to form a first mixture; (b) exposing the firstmixture to a reaction promoter; (c) heating the first mixture to atemperature that is sufficiently high to form nanoparticles, i.e.,nucleation crystals, when a second precursor is added; (d) introducing asecond precursor into the first mixture to form a second mixture therebyresulting in the formation of a plurality of nanoparticles; and (e)cooling the second mixture to stop further growth of the nanoparticles.A reducing agent or oxygen source serves as the reaction promoter.

The exposure to a reaction promoter provides for a higher yield ofnanoparticles, typically up to three times more yield, more preferablyup to 19 times better yield when compared to those methods where noreaction promoter is used. In addition, the nanoparticle yield can bemodulated by modulating the amount of reaction promoter added for thelength of time of exposure.

The methods described herein can provide nanoparticles having an averageparticle diameter within the range of about 1.5 to 15 Å, with a particlesize deviation of less than about 10% rms in diameter.

The methods described herein are particularly useful for preparing amonodisperse population of CdSe nanoparticles having an emission peakwavelength that is preferably less than about 570 nm, preferably lessthan about 520 nm, more preferably less than about 500 nm.

In addition, the methods described herein can provide a monodispersepopulation of nanoparticles having an emission peak wavelength that isless than about 35 nm at full width at half max (FWHM), preferably lessthan about 30 nm FWHM, more preferably less than about 25 nm FWHM.

I. Definitions and Nomenclature

Before describing detailed embodiments of the invention, it is to beunderstood that unless otherwise indicated, this invention is notlimited to specific nanoparticle materials or manufacturing processes,as such may vary. It may be useful to set forth definitions that areused in describing the invention. The definitions set forth apply onlyto the terms as they are used in this patent and may not be applicableto the same terms as used elsewhere, for example in scientificliterature or other patents or applications including other applicationsby these inventors or assigned to common owners. The followingdescription of the preferred embodiments and examples are provided byway of explanation and illustration, and is not intended to be limiting.

It must be noted that, as used in this specification and the appendedclaims, the singular forms “a”, “an” and “the” include plural referentsunless the context clearly dictates otherwise. Thus, for example,reference to “a nanoparticle” includes a single nanoparticle as well astwo or more nanoparticles, and the like.

In describing and claiming the present invention, the followingterminology will be used in accordance with the definitions set outbelow.

The term “nanoparticle” refers to a particle, generally a semiconductorparticle, having a diameter in the range of about 1-1000 nm, preferablyin the range of about 2-50 nm, more preferably in the range of about2-20 nm.

The terms “semiconductor nanoparticle” and “semiconductive nanoparticle”refer to a nanoparticle as defined herein, that is composed of aninorganic semiconductive material, an alloy or other mixture ofinorganic semiconductive materials, an organic semiconductive material,or an inorganic or organic semiconductive core contained within one ormore semiconductive overcoat layers.

The terms “semiconductor nanocrystal,” “quantum dot” and “Qdot™nanocrystal” are used interchangeably herein to refer to semiconductornanoparticles composed of an inorganic crystalline material that isluminescent (i.e., they are capable of emitting electromagneticradiation upon excitation), and include an inner core of one or morefirst semiconductor materials that is optionally contained within anovercoating or “shell” of a second inorganic material. A semiconductornanocrystal core surrounded by an inorganic shell is referred to as a“core/shell” semiconductor nanocrystal. The surrounding shell materialwill preferably have a bandgap energy that is larger than the bandgapenergy of the core material and may be chosen to have an atomic spacingclose to that of the core substrate.

The term “solid solution” is used herein to refer to a compositionalvariation that is the result of the replacement of ions or ionic groupswith other ions or ionic groups, e.g., CdS in which some of the Cd atomshave been replaced with Zn. This is in contrast to a “mixture,” a subsetof which is an “alloy,” which is used herein to refer to a class ofmatter with definite properties whose members are composed of two ormore substances, each retaining its own identifying properties.

By “luminescence” is meant the process of emitting electromagneticradiation (light) from an object. Luminescence results when a systemundergoes a transition from an excited state to a lower energy statewith a corresponding release of energy in the form of a photon. Theseenergy states can be electronic, vibrational, rotational, or anycombination thereof. The transition responsible for luminescence can bestimulated through the release of energy stored in the system chemicallyor added to the system from an external source. The external source ofenergy can be of a variety of types including chemical, thermal,electrical, magnetic, electromagnetic, and physical, or any other typeof energy source capable of causing a system to be excited into a statehigher in energy than the ground state. For example, a system can beexcited by absorbing a photon of light, by being placed in an electricalfield, or through a chemical oxidation-reduction reaction. The energy ofthe photons emitted during luminescence can be in a range fromlow-energy microwave radiation to high-energy x-ray radiation.Typically, luminescence refers to photons in the range from UV to IRradiation, and usually refers to visible electromagnetic radiation(i.e., light).

The term “monodisperse” refers to a population of particles (e.g., acolloidal system) wherein the particles have substantially identicalsize and shape. For the purpose of the present invention, a“monodisperse” population of particles means that at least about 60% ofthe particles, preferably about 75-90% of the particles, fall within aspecified particle size range. A population of monodisperse particlesdeviates less than 10% rms (root-mean-square) in diameter and preferablyless than 5% rms.

The phrase “one or more sizes of nanoparticles” is used synonymouslywith the phrase “one or more particle size distributions ofnanoparticles.” One of ordinary skill in the art will realize thatparticular sizes of nanoparticles such as semiconductor nanocrystals areactually obtained as particle size distributions.

By use of the term “narrow wavelength band” or “narrow spectrallinewidth” with regard to the electromagnetic radiation emission of thesemiconductor nanocrystal is meant a wavelength band of emissions notexceeding about 60 nm, and preferably not exceeding about 30 nm inwidth, more preferably not exceeding about 20 nm in width, and symmetricabout the center. It should be noted that the bandwidths referred to aredetermined from measurement of the full width of the emissions at halfpeak height (FWHM), and are appropriate in the emission range of200-2000 nm.

By use of the term “a broad wavelength band,” with regard to theexcitation of the semiconductor nanocrystal is meant absorption ofradiation having a wavelength equal to, or shorter than, the wavelengthof the onset radiation (the onset radiation is understood to be thelongest wavelength (lowest energy) radiation capable of being absorbedby the semiconductor nanocrystal). This onset occurs near to, but atslightly higher energy than the “narrow wavelength band” of theemission. This is in contrast to the “narrow absorption band” of dyemolecules, which occurs near the emission peak on the high energy side,but drops off rapidly away from that wavelength and is often negligibleat wavelengths further than 100 nm from the emission.

The term “emission peak” refers to the wavelength of light within thecharacteristic emission spectra exhibited by a particular semiconductornanocrystal size distribution that demonstrates the highest relativeintensity.

The term “alkyl” as used herein refers to a branched or unbranchedsaturated hydrocarbon group of 1 to approximately 24 carbon atoms, suchas methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, t-butyl,octyl, decyl, tetradecyl, hexadecyl, eicosyl and tetracosyl, as well ascycloalkyl groups such as cyclopentyl and cyclohexyl. Similarly, alkanesare saturated hydrocarbon compounds such as methane, ethane, and soforth. The term “lower alkyl” is intended to mean an alkyl group of 1 to4 carbon atoms, and thus includes methyl, ethyl, n-propyl, isopropyl,n-butyl, isobutyl and t-butyl.

The term “alkene” as used herein refers to a branched or unbranchedhydrocarbon compound typically although not necessarily containing 2 toabout 24 carbon atoms and at least one double bond, such as ethylene,n-propylene, isopropylene, butene, butylene, propylene, octene,decylene, and the like. Generally, although not necessarily, the alkenesused herein contain 2 to about 29 carbon atoms, preferably about 8 toabout 20 carbon atoms. The term “lower alkene” is intended to mean analkene of 2 to 4 carbon atoms.

The term “alkyne” as used herein refers to a branched or unbranchedhydrocarbon group typically although not necessarily containing 2 toabout 24 carbon atoms and at least one triple bond, such as acetylene,allylene, ethyl acetylene, octynyl, decynyl, and the like. Generally,although again not necessarily, the alkynes used herein contain 2 toabout 12 carbon atoms. The term “lower alkyne” intends an alkyne of 2 to4 carbon atoms, preferably 3 or 4 carbon atoms.

II. Precursors

There are numerous inorganic materials that are suitable for use asmaterials for the core and/or shell of semiconductor nanoparticles.These include, by way of illustration and not limitation, materialscomprised of a first element selected from Groups 2 and 12 of thePeriodic Table of the Elements and a second element selected from Group16 (e.g., ZnS, ZnSe, ZnTe, CdS, CdSe, CdTe, HgS, HgSe, HgTe, MgS, MgSe,MgTe, CaS, CaSe, CaTe, SrS, SrSe, SrTe, BaS, BaSe, BaTe, and the like);materials comprised of a first element selected from Group 13 of thePeriodic Table of the Elements and a second element selected from Group15 (GaN, GaP, GaAs, GaSb, InN, InP, InAs, InSb, and the like); ternaryand quaternary mixtures comprised of a Group 14 element (Ge, Si, and thelike); materials comprised of a first element selected from Group 14element of the Periodic Table of the Elements and a second elementselected from Group 16 (e.g., PbS, PbSe and the like); materialscomprised of a first element selected from Group 13 of the PeriodicTable of the Elements and a second element selected from Groups 15 and16 (e.g., AlS, AlP, AlSb, and the like); and alloys and mixturesthereof. As used herein, all reference to the Periodic Table of theElements and groups thereof is to the new IUPAC system for numberingelement groups, as set forth in the Handbook of Chemistry and Physics,81^(st) Edition (CRC Press, 2000).

The selection of the composition of the semiconductor nanoparticleaffects the characteristic spectral emission wavelength of thesemiconductor nanocrystal. Thus, as one of ordinary skill in the artwill realize, a particular composition of a nanoparticle of theinvention will be selected based upon the spectral region beingmonitored. For example, semiconductor nanocrystals that emit energy inthe visible range include, but are not limited to, CdS, CdSe, CdTe,ZnSe, ZnTe, GaP, and GaAs. Semiconductor nanocrystals that emit energyin the near IR range include, but are not limited to, InP, InAs, InSb,PbS, and PbSe. Finally, semiconductor nanocrystals that emit energy inthe blue to near-ultraviolet include, but are not limited to, ZnS andGaN.

Precursors useful as the “first” precursor in the methods of theinvention include compounds containing elements from Groups 2 and 12 ofthe Periodic Table of the Elements (e.g., Zn, Cd, Hg, Mg, Ca, Sr, Ba,and the like), compounds containing elements from Group 13 of thePeriodic Table of the Elements (Al, Ga, In, and the like), and compoundscontaining elements from Group 14 of the Periodic Table of the Elements(Si, Ge, Pb, and the like).

Precursors useful as the “second” precursor in the methods of theinvention include compounds containing elements from Group 16 of thePeriodic Table of the Elements (e.g., S, Se, Te, and the like),compounds containing elements from Group 15 of the Periodic Table of theElements (N, P, As, Sb, and the like), and compounds containing elementsfrom Group 14 of the Periodic Table of the Elements (Ge, Si, and thelike).

Many forms of the precursors can be used in the methods of theinvention. Suitable element-containing compounds useful as the firstprecursor, can be organometallic compounds such as Cd(CH₃)₂, oxides suchas CdO, halogenated compounds such as CdCl₂, and other salts such ascadmium acetate.

Suitable second precursors include tri-n-alkylphosphine adducts such astri-n-(butylphosphine)selenide (TBPSe) andtri-n-(octylphosphine)selenide (TOPSe), hydrogenated compounds such asH₂Se, silyl compounds such as bis(trimethylsilyl)selenium ((TMS)₂Se),and metal salts such as NaHSe. These are typically formed by combining adesired element, such as Se, with an appropriate coordinating solvent,e.g., TOP. Other exemplary organic precursors are described in U.S. Pat.Nos. 6,207,299 and 6,322,901 to Bawendi et al., and synthesis methodsusing weak acids as precursor materials are disclosed by Qu et al.,(2001) “Alternative Routes toward High Quality CdSe Nanocrystals,” NanoLett., 1(6):333-337, the disclosures of which are incorporated herein byreference.

Both the first and the secondary precursor can be combined with anappropriate coordinating solvent to form a solution for use in themethods of the invention. The coordinating solvent used to form a firstprecursor solution may be the same or different from that used to form asecond precursor solution.

III. Coordinating Solvent

Suitable coordinating reaction solvents include, by way of illustrationand not limitation, amines, alkyl phosphines, alkyl phosphine oxides,fatty acids, ethers, furans, phospho-acids, pyridines, alkenes, alkynesand combinations thereof. The solvent may actually comprise a mixture ofsolvents, often referred to in the art as a “solvent system”.Furthermore, the coordinating solvent might be a mixture of anessentially non-coordinating solvent such as an alkane and a ligand asdefined below.

Suitable amines include, but are not limited to, alkylamines such adodecylamine and hexyldecylamine, and so forth.

Exemplary alkyl phosphines include, but are not limited to, the trialkylphosphines, tri-n-butylphosphine (TBP), tri-n-octylphosphine (TOP), andso forth.

Suitable alkyl phosphine oxides include, but are not limited to, thetrialkyl phosphine oxide, tri-n-octylphosphine oxide (TOPO), and soforth.

Exemplary fatty acids include, but are not limited to, stearic andlauric acids. It will be appreciated that the rate of nanocrystal growthgenerally increases as the length of the fatty acid chain decreases.

Exemplary ethers and furans include, but are not limited to,tetrahydrofuran and its methylated forms, glymes, and so forth.

Suitable phospho-acids include, but are not limited to hexylphosphonicacid, tetradecylphosphonic acid, and octylphosphinic acid, and arepreferably used in combination with an alkyl phosphine oxide such asTOPO.

Exemplary pyridines include, but are not limited to, pyridine, alkylatedpyridines, nicotinic acid, and so forth.

Coordinating solvents can be used alone or in combination. TOP-TOPOsolvent systems are commonly utilized in the art, as are other related(e.g., butyl) systems. For example, TOP and TOPO can be used incombination to form a cadmium solution, while TOP, alone, can be used toform a selenium solution.

Technical grade coordinating solvents can be used, and benefits can beobtained from the existence of beneficial impurities in such solvents,e.g. TOP, TOPO or both. However, in one preferred embodiment, thecoordinating solvent is pure. Typically this means that the coordinatingsolvent contains less than 10 vol %, and more preferably less than 5 vol% of impurities that can function as reductants. Therefore, solventssuch as TOPO at 90% or 97% purity and TOP at 90% purity are particularlywell suited for use in the methods of the invention.

IV. Ligand

In one preferred embodiment, ligands are included in the reaction.Ligands are compounds that complex with a precursor and/or ananoparticle. Suitable ligands include, by way of illustration and notlimitation, phospho-acids such as hexylphosphonic acid andtetradecylphosphonic acid, carboxylic acids such as isomers ofoctadecanoic acid, amines, amides, alcohols, ethers, alkenes, andalkynes. In some cases, the ligand and the solvent can be the same.

V. Reaction Promoter

The methods of the invention, to some extent, are based upon the premisethat particle growth kinetics are strongly impacted by the effectivenessof the initial nucleation events and that the chemical reduction of oneof the precursors is expected to be an important rate-determiningfactor. This is in contrast with state of the art metholodologies thatoperate under the assumption that precursor/particle sequestrationevents and the precursor-injection temperature drop dominates thenucleation/growth temporal interface.

The reaction promoter increases the reactivity of the nanoparticleprecursors in such a way so as to allow control of the nucleationprocess, or growth process, or both. In the methods of the invention,the reactants are exposed to the reaction promoter in a carefullycontrolled manner, typically by physically adding the reaction promoterto the mixture. This serves to provoke and modulate increasedreactivity.

In the fast kinetic growth regime, nanoparticles can grow rapidly whenthe concentration of monomer precursors is high relative to the numberof particles. Such growth is accompanied by narrowing of the particlesize distribution. As long as this condition exists, the particle sizedistribution can remain focused. When the monomer concentration isreduced to a level that cannot maintain the optimum growth rate,statistical broadening of size distributions is generally observed.Optimally, the reaction should be stopped prior to the occurrence ofsuch defocusing so as to ensure optimally narrow particle sizedistributions. The methods of the invention achieve this by controllingthe number of nuclei formed. The initial reactivity is controlled tomake fewer nuclei and to obtain larger particles. When smaller particlesare desired, reactivity can be boosted to produce more nuclei. In eithercase, the reaction is stopped at the point where growth begins to slowdue to monomer depletion. This corresponds to the maximum practicalyield for the chosen particle size, while not sacrificing narrowparticle size distribution.

The reaction promoter can be an oxygen source or a reducing agent. Whilenot wishing to be bound by theory, there are several ways in which theoxygen reaction promoter, for example, may be functioning in the methodsdescribed herein. First, in the presence of O₂, the initial nuclei inthe core reaction may not be CdSe, but rather CdO (or CdOH). Thesenuclei form more easily than the CdSe nuclei for a variety of reasons,such as issues of driving force, activation, and differentialsequestration of precursors. These cores can provide a growth site forCdSe. During the process, the oxygen atoms can be annealed out or canremain at the core of the final material. Second, some impurities may bepresent in the reaction, for example from the TOP. These impurities mayhinder the growth of particles through sequestration of redoxreactivity. In this case, the oxygen in the reaction may be responsiblefor destroying the impurities and thus indirectly facilitating thereaction.

It is preferable to use two reducing equivalents per precursor (e.g.,cadmium) equivalent to prepare the nanoparticles when salt feedstocksare utilized (or created in situ through, for example acid-basereactions). Oxygen may facilitate these redox reactions directly orindirectly through the formation of some intermediates. An example ofthis indirect mechanism involves initial oxidation of TOP to a speciessuch as di-octyl-octylphosphinate. This species might disproportionateto form di-octylphosphine and octyl-di-octylphosphonate by the followingscheme:

Similar chemistries are possible with other oxidized impurities such asdi-octylphosphine oxide. The resulting secondary (or primary) phosphinesare potent reductants that would be expected to enhance the reactionrates as observed. This last mechanism supports the direct addition ofsuch phosphines as reducing agents as described below. Addition ofoxidants and/or proton carriers other than oxygen/water can provide aneven more serviceable approach to taking full advantage of this effect.

FIG. 1 and FIG. 2 show the effect on temporal wavelength evolutionduring the course of a CdSe nanocrystals core-forming reaction as afunction of added reaction promoter. The line labeled “Control” is thestandard reaction with no added reaction promoter. The line labeled “Airadded” shows the effects of exposure of the reaction to air on peakemission wavelength and emission FWHM. The remaining three lines depictthe reaction course when equivalent amounts of reducing agent reactionpromoters are used, specifically, dicyclohexylphosphine, DPP, andhydroquinone. FIG. 1 demonstrates that both an oxygen source as well asreducing agents are suitable for use as reaction promoters in themethods of the invention. The key characteristics of these plots are thelength of the pre-nucleation induction, the initial nuclei size, thestall wavelength for growth, and the evolution of the degree ofdispersity (as estimated by emission FWHM).

FIG. 3 and FIG. 4 illustrate the use of increasing amounts of thereaction promoter, DPP. The concentrations are given as number ofequivalents relative to cadmium added to the core reactions. The amountof reaction promoter that is added can be used to tune the properties ofthe reactions including the yield of particles.

FIG. 5 illustrates the particle counts recovered from each of thereactions shown in FIG. 3 and illustrates the level of control affordedby the addition of reaction promoters.

1. Reducing Agents

The reducing agent functions to provide electrons (reducing equivalents)to the reactants or reactant mixture. Phosphine-based reductants are apreferred class of reducing agents for use in the methods of theinvention. However, non-phosphine, non-ligating chemical reductants suchas hydroquinone are also suitable to provide the necessary reducingequivalents. Accordingly, suitable reducing agents include, by way ofillustration and not limitation, chemical compounds such as tertiary,secondary, and primary phosphines (e.g., diphenylphosphine,dicyclohexylphosphine, and diotylphosphine); amines (e.g., decyl- andhexadecylamine); hydrazines; hydroxyphenyl compounds (e.g., hydroquinoneand phenol); hydrogen; hydrides (e.g., sodium borohydride, sodiumhydride and lithium aluminum hydride); metals (e.g., mercury andpotassium); boranes (e.g., THF:BH₃ and B₂H₆); aldehydes (e.g.,benzaldehyde and butyraldehyde); alcohols and thiols (e.g., ethanol andthioethanol); reducing halides (e.g., I″ and I₃ ⁻); polyfunctionalreductant versions of these species (e.g., a single chemical speciesthat contains more than one reductant moiety, each reductant moietyhaving the same or different reducing capacity, such astris-(hydroxypropyl)phosphine and ethanolamine); and so forth.

In addition, it is expected that there may be particular advantagesassociated with the use of an electrochemical system (cathode-anodesystem) as the reducing agent, i.e., the cathode would serve as a sourceof electrons. By utilizing an electrode as a source of reducingequivalents, coulombic equivalents can be readily counted and their rateof delivery directly controlled. Use of electrodes also allows forcontrolling both the physical localization of reduction events, as wellas the potential for direct formation of particle arrays at theelectrode surface. Since the cathode will be positioned within thereaction chamber, the material selection is preferably one that will notreact with the precursors, ligand or coordinating solvents. The anode,will typically be positioned outside of the reaction vessel so materialselection is not limited and any well known anode material can be used.Exemplary cathode materials include platinum, silver, or carbon. Anexemplary method for delivering reducing equivalents to the cathodeincludes the use of a constant current or potentiostat in two- (workingand counter) or three-electrode (working, counter, and reference)configuration.

2. Oxygen

The reaction promoter can also be any source of oxygen. In oneembodiment of the invention, the reaction promoter is an air stream,preferably dry. In this embodiment, the mixture is exposed directly tothe oxygen or oxygen source.

The oxygen can also be formed in situ. Accordingly, in anotherembodiment, the precursors are exposed to compounds which provide thesame effect as the aforementioned controlled exposure to air. Forexample, oxygen can be formed by a redox reaction. Therefore, since themechanism by which oxygen enhances the process could include a reductionstep, reducing agents can added directly to utilize redox reactivity.Suitable reducing agents are those described above.

Previous methods for the high temperature synthesis of nanoparticles useair-free conditions due to the combustive nature of the reactionprecursors. However, the methods of the invention provide for exposingthe reactants to the oxygen reaction promoter in such a way so as toreduce or eliminate this combustion hazard.

VI. Methods of Producing Nanoparticles

The methods described herein find utility in producing a variety ofnanoparticles, including metal-chalcogen nanoparticles such as CdSe,CdTe, CdS, ZnS, ZnSe, and so forth.

One embodiment of the invention is a method of producing nanoparticlescomprising: (a) mixing a first precursor and at least one coordinatingsolvent to form a first mixture; (b) exposing the first mixture to areaction promoter selected from the group consisting of oxygen and areducing agent; (c) heating the first mixture to a temperature that issufficiently high to form nanoparticles (nucleation crystals) when asecond precursor is added; (d) introducing a second precursor into thefirst mixture to form a second mixture thereby resulting in theformation of a plurality of nanoparticles; and (e) cooling the secondmixture to stop further growth of the nanoparticles.

In one embodiment, the reaction promoter is a reducing agent and theexposing step can involve adding a chemical reducing agent to themixture or the mixture can be exposed to an appropriate electrodereducing agent. In another embodiment, the exposure step involvesdirectly exposing the mixture to a source of oxygen. The oxygen can befrom an external source or it can be created in situ. In yet anotherembodiment, the exposing step involves exposing a coordinating solventto a source of oxygen and then adding this exposed coordinating solventto the mixture.

The mixing step is typically conducted at an elevated temperature or thereaction mixture is heated to an elevated temperature while mixing. Thiselevated temperature is commonly within the range of about 150 to 350°C. In addition, the mixing step can be conducted in a vessel that isevacuated and filled and/or flushed with an inert gas such as nitrogen.The filling can be periodic or the filling can occur, followed bycontinuous flushing for a set period of time. The mixing step caninvolve a cooling step prior to exposure to the reaction promoter, forexample, cooling to a temperature within the range of about 50 to 150°C., typically about 100° C.

The exposing step was described above with regard to the reactionpromoter, and can be conducted either at an elevated temperature (e.g.,150 to 350° C.) or at a reduced temperature (e.g., 50 to 150° C.). Inaddition, the reaction promoter can be heated to a temperature such aswithin the range of about 50 to 150° C., prior to being added to themixture.

The heating step is done at a temperature that is sufficient to inducetemporally discrete homogeneous nucleation, which results in theformation of a monodisperse population of individual nanoparticles.Typically, this heating step achieves a temperature within the range ofabout 150-350° C., more preferably within the range of about 250-350° C.

It is understood, however, that the above ranges are merely exemplaryand are not intended to be limiting in any manner as the actualtemperature ranges may vary, dependent upon the relative stability ofthe reaction promoter, precursors, ligands and coordinating solvents.

The introducing step may be an injection step, which typically involvesapplying pressure to the second precursor so that a fluid stream can beinjected into the heated mixture. Pressure can be applied in numerousways, for example by means of a pressurized inert gas, a syringe, apumping means, and so forth, as well as combinations thereof. Theresulting mixture may be heated so as to maintain the elevatedtemperature. Thus, the introducing step is conducted at a temperaturewithin the range of about 150-350° C., more preferably within the rangeof about 250-270° C. The introducing step can be carried out in onerapid step or slowly over time.

The secondary precursor is typically combined with an appropriatecoordinating solvent to form a solution for use in the method of theinvention. This coordinating solvent may be the same or different fromthat used in combination with the first precursor.

The cooling step typically achieves a temperature within the range ofabout 50-150° C., more preferably within the range of about 90-110° C.However, the actual temperature range may vary, dependent upon therelative stability of the reaction promoter, precursors, ligands andcoordinating solvents.

Size distribution during the growth stage of the nanoparticles can beapproximated by monitoring the emission of a particle sampling.

Exemplary embodiments are set forth below, as well as in the examples.

In one exemplary embodiment, nanoparticles are produced by a methodwhere the first precursor is exposed to the reaction promoter, oxygen. Acadmium solution is prepared by first dissolving anhydrous cadmiumacetate in TOP, and this mixture is then mixed with TOPO, TDPA andadditional TOP. Dry air is then injected into the reaction vessel so asto expose the mixture to oxygen. The duration of exposure can be from1-10 minutes, or longer. Heating is then done for a sufficient time andtemperature so as to insure formation of nanoparticles when the secondprecursor is added, for example, heating to 270° C. A selenium solution,previously prepared by dissolving Se in TOP, is then introduced byinjection into the cadmium solution, thus forming CdSe nanoparticles.The reaction is stopped by cooling.

In another exemplary embodiment, nanoparticles are produced by a methodwhere a coordinating solvent is exposed to oxygen and then added to thefirst precursor. A cadmium solution is prepared by first dissolvinganhydrous cadmium acetate in TOP, and this mixture is then mixed withTOPO, TDPA and additional TOP. The mixture is heated to a temperaturesufficiently high to insure formation of nanoparticles when the secondprecursor is added, and the elevated temperature maintained. In aseparate container, TOP is heated and exposed to air. The duration ofexposure can be for a period of between 10 minutes and 48 hours,preferably between 30 minutes and 24 hours, and even more preferably 50minutes to 2 hours. The air-exposed TOP is then added to the cadmiumsolution. A selenium solution, previously prepared by dissolving Se inTOP, is then introduced by injection into the cadmium solution, thusforming CdSe nanoparticles. The reaction is stopped by cooling.

Another exemplary embodiment, pertains to the production ofnanoparticles by a method where the first precursor is exposed to achemical reducing agent reaction promoter. A cadmium solution isprepared by first dissolving anhydrous cadmium acetate in TOP, and thismixture is then mixed with TOPO and TDPA. A chemical reducing agent suchas dicyclohexylphosphine, diphenylphosphine or hydroquinone is thenadded to the mixture, and the mixture heated to a temperaturesufficiently high to insure formation of nanoparticles when the secondprecursor is added. A selenium solution, previously prepared bydissolving Se in TOP, is then introduced by injection into the cadmiumsolution, thus forming CdSe nanoparticles. The reaction is stopped bycooling.

Another exemplary embodiment, pertains to the production ofnanoparticles by a method wherein the addition of a reaction promoter isused to induce nucleation. A precursor solution is prepared by firstdissolving anhydrous cadmium acetate in TOP, and this mixture is thenmixed with TOPO and TDPA and a TOP solution of Se. The mixture is heatedto a temperature sufficiently high to insure formation of nanoparticleswhen the promoter is added. Nucleation and subsequent growth ofnanoparticles is then induced by introduction of a suitable reactionpromoter. For example, a chemical reducing agent reaction promoter suchas dicyclohexylphosphine, diphenylphosphine or hydroquinone is thenintroduced by injection into the mixture, thus forming CdSenanoparticles. The reaction is stopped by cooling.

Another exemplary embodiment, illustrated in FIG. 8, pertains to theproduction of nanoparticles by a method where the first precursor isexposed to an electrode reducing agent reaction promoter. The system 10includes a reaction vessel 12 having first 14 and second 16 compartmentsseparated by an ion permeable barrier 18. A cadmium solution is preparedby first dissolving anhydrous cadmium acetate in TOP, and this mixtureis then mixed with TOPO and TDPA. The resulting mixture 20 is placed incompartment 16. An electrochemical system reaction promoter, such as aplatinum cathode 22 is then immersed in mixture 20. A platinum anode 24would be set up immersed in a solution containing an oxidizablecomponent (e.g., iodide) 26 in compartment 14 and an appropriate powersupply 28 would be set up outside the reaction vessel, and in electricalcommunication with cathode 22 and anode 24 through leads 32 and 34,respectively. Optionally, magnetic stirring bars 30 can be placed incompartments 14 and 16. The mixture is heated to a temperaturesufficiently high to insure formation of nanoparticles when the secondprecursor is added at an appropriate potential. A selenium solution,previously prepared by dissolving Se in TOP, is then injected into thecadmium solution 20 in compartment 16. A negative potential is appliedat the cathode 22 to induce formation of nanoparticles. The reaction isstopped by cooling, and/or by removing the potential.

VII. Methods of Forming Nanoparticle Shells

The surface of the semiconductor nanoparticle can be modified to enhancethe efficiency of the emissions, by adding an inorganic layer or shell.The overcoating layer can be particularly useful since surface defectson the semiconductor nanoparticle can result in traps for electrons orholes that degrade the electrical and optical properties of thenanoparticle. An insulating layer at the surface of the nanoparticleprovides an atomically abrupt jump in the chemical potential at theinterface that eliminates energy states that can serve as traps for theelectrons and holes. This results in higher efficiency in theluminescent process.

The nanoparticles produced by the methods described herein can beprovided with a shell by any method known in the art. See for example,Dabbousi et al., J. Phys. Chem. B 101:9463 (1997), Hines et al., J.Phys. Chem. 100:468-471 (1996), Peng et al., J. Am. Chem. Soc.119:7019-7029 (1997), and Kuno et al., J. Phys. Chem. 106:9869 (1997).In addition, the nanoparticles of the invention can also be providedwith a shell using the reaction promoter-based method described herein.In fact, the reaction promoter-based methods of the invention findutility in nanoparticle shell procedures for both nanoparticle coresproduced by the methods described herein as well as nanoparticle coresproduced by other methods.

The shell can have a thickness within the range of about 1-100 nm, andis preferably within the range of about 2-10 nm thick.

Suitable materials for the inorganic shell layer include semiconductormaterials having a higher bandgap energy than the semiconductornanoparticle core. In addition to having a bandgap energy greater thanthe core, suitable materials for the shell should have good conductionand valence band offset with respect to the core. Thus, the conductionband is desirably higher and the valence band is desirably lower thanthose of the core. For a semiconductor nanoparticle core that emitsenergy in the visible (e.g., CdS, CdSe, CdTe, ZnSe, ZnTe, GaP, GaAs) ornear IR (e.g., InP, InAs, InSb, PbS, PbSe) range, materials that have abandgap energy in the ultraviolet regions may be used. Exemplarymaterials include ZnS, GaN, and magnesium chalcogenides, e.g., MgS,MgSe, and MgTe. For a semiconductor nanoparticle core that emits in thenear IR range, materials having a bandgap energy in the visible range,such as CdS or CdSe, may also be used.

In addition, a passivation layer of the desired thickness can also beeasily introduced onto semiconductor nanoparticles of the invention byintroducing appropriate solvents and/or surfactants during thenanoparticle manufacture. For example, semiconductive nanoparticles, asmanufactured by the methods described herein, can be provided with awater-insoluble organic overcoat that has an affinity for thesemiconductive core material. This coating will typically be apassivating layer produced by one or more coordinating solvents asdescribed above, e.g., hexyldecylamine, TOPO, TOP, TBP, and so forth; orproduced by one or more hydrophobic surfactants such as, by way ofexample, octanethiol, dodecanethiol, dodecylamine, tetraoctylammoniumbromide, and so forth, as well as combinations thereof.

Further, as noted above, the nanoparticle shell can be produced by thereaction promoter-based methods of the invention. Accordingly, anembodiment of the invention is a method of producing a nanoparticleshell comprising: producing nanoparticles using steps (a) through (e)described above in Section VI; (a′) mixing the nanoparticles with atleast one coordinating solvent to form a third mixture; (b′) heating thethird mixture to a temperature that is sufficiently high to form a shellon the nanoparticles when third and fourth precursors are added; (c′)introducing third and fourth precursors into the third mixture to form afourth mixture thereby resulting in the formation of shells on aplurality of nanoparticles; and (d′) cooling the third mixture to stopfurther growth of the shell; wherein the method further comprisesexposing the third or fourth mixture to a reaction promoter selectedfrom the group consisting of oxygen and a reducing agent. Exposure tothe reaction promoter can occur at several stages during the shellproduction method. For example, the third mixture can be exposed to thereaction promoter after step (a′), the heated third mixture can beexposed to the reaction promoter after step (b′), or the fourth mixturecan be exposed to the reaction promoter in step (c′). In a preferredembodiment, the third mixture is exposed to the reaction promoter afterstep (a′).

As noted above, the semiconductor materials used in the shell preferablyhave a higher bandgap energy than the semiconductor nanoparticle core.Therefore, the precursors used in steps (d) (“first” and “second”precursors) are preferably different than the precursors used in step(c′) (“third” and “fourth” precursors). However, the ligand (if used)and coordinating solvents used in step (a′) can be the same or differentfrom those used in step (a).

In addition, the shell producing method of the invention also findsutility for nanoparticle cores produced by any method known in the art.Accordingly, another embodiment of the invention is a method ofproducing nanoparticle shells comprising (a) mixing nanoparticles withat least one coordinating solvent to form a first mixture; (b) heatingthe first mixture to a temperature that is sufficiently high to form ashell on the nanoparticles when first and second precursors are added;(c) introducing first and second precursors into the first mixture toform a second mixture thereby resulting in the formation of shells on aplurality of nanoparticles; and (d) cooling the second mixture to stopfurther growth of the shell; wherein the method further comprisesexposing the first or second mixture to a reaction promoter selectedfrom the group consisting of oxygen and a reducing agent. As notedabove, exposure to the reaction promoter can occur at several stagesduring the shell producing method. For example, the first mixture can beexposed to the reaction promoter after step (a), the heated firstmixture can be exposed to the reaction promoter after step (b), or thesecond mixture can be exposed to the reaction promoter in step (c). In apreferred embodiment, the first mixture is exposed to the reactionpromoter after step (a).

VIII. Shell Additives

The methods of the invention also find utility in nanoparticles asdescribed in commonly owned, co-pending U.S. patent application Ser. No.10/198,635, filed on Jul. 17, 2002, by Treadway et al., the disclosureof which in incorporated herein in its entirety, for both nanoparticlesproduced by the methods described herein or produced by state of the artmethods.

Accordingly, an embodiment of the invention is a method of producingnanoparticles comprising: producing nanoparticles using steps (a) though(e) described above or produced by any state of the art method; and (a′)mixing the nanoparticles with an additive or additive precursor, anoptional ligand and at least one coordinating solvent to form a thirdmixture; (b′) exposing the third mixture to a reaction promoter selectedfrom the group consisting of oxygen and a reducing agent; (c′) heatingthe third mixture to a temperature that is sufficiently high to form ashell on the nanoparticles when third and fourth precursors are added;(d′) introducing third and fourth precursors into the third mixture toform a fourth mixture thereby resulting in the formation of shells on aplurality of nanoparticles; and (e′) cooling the fourth mixture to stopfurther growth of the shell. The additive or additive precursor can beany inorganic material that is suitable for use in the manufacture ofsemiconductor nanoparticles, such as those described herein.

The shell-forming aspect of the inventions as described herein isillustrated as follows. InAs nanoparticles are produced by the methodsdescribed herein or by methods that are well known in the art. Thenanoparticles are then mixed with an additive precursor (e.g., a sourceof In⁺³), an optional ligand and at least one coordinating solvent toform a mixture. The mixture is then exposed to a reaction promoter suchas DPP. The mixture is heated to the appropriate temperature and firstand second precursors (e.g., a source of Cd²⁺ and Se⁰) are introduced byinjection into the mixture to form a CdSe shell. The mixture is thencooled to stop further growth of the shell. Since the Cd⁺² must bereduced to Cd⁰ and the In⁺³ must be reduced to In⁰, at least 5equivalents of the reaction promoter would be optimal, depending, atleast in part, on the relative amounts of additive (In⁺³) and firstprecursor (Cd²⁺).

IX. Methods of Overcoating Nanoparticles

The nanoparticles of the invention may also be provided with an organiccoating. Suitable organic materials include agaroses; cellulose;epoxies; and polymers such as polyacrylamide, polyacrylate,poly-diacetylene, polyether, polyethylene, polyimidazole, polyimide,polypeptides, polyphosphate, polyphenylene-vinylene, polypyrrole,polysaccaride, polystyrene, polysulfone, polythiophene, and polyvinyl.The coating can also be a material such as silica glass; silica gel;siloxane; and the like.

Therefore, the invention also encompasses a method of producing coatednanoparticles comprising: producing nanoparticles, wherein thenanoparticle core and/or shell is produced by the methods of theinvention; and mixing the nanoparticles with an organic compound havingaffinity for the nanoparticle surface, whereby the organic compounddisplaces the coordinating solvent to form a coating on the nanoparticlesurface. The organic coating step is preferably conducted at atemperature within the range of about 50 to 350° C., preferably withinthe range of about 150 to 250° C. The actual temperature range of thecoating step may vary, dependent upon the relative stability of thereaction promoter, precursors, ligands, coordinating solvents andoverlayer composition.

EXAMPLES

The practice of the present invention will employ, unless otherwiseindicated, conventional techniques of synthetic inorganic, organicchemistry, chemical engineering, and the like, which are within theskill of the art. Such techniques are explained fully in the literature.See, for example, Kirk-Othmer's Encyclopedia of Chemical Technology,House's Modern Synthetic Reactions; and the Chemical Engineer'sHandbook.

The following examples are put forth so as to provide those of ordinaryskill in the art with a complete disclosure and description of how tomake and use the compositions and methods of the invention. Efforts havebeen made to ensure accuracy with respect to numbers (e.g., amounts,temperature, etc.) but some experimental error and deviations should, orcourse, be allowed for. Unless indicated otherwise, parts are parts byweight, temperature is degrees centrigrade and pressure is at or nearatmospheric. All components were obtained commercially unless otherwiseindicated.

Materials

In all the examples which follow, materials were obtained as follows,unless otherwise indicated: tri-n-octylphosphine oxide (TOPO, >97%purity) was from Fluka; tri-n-octylphosphine (TOP, 90% purity) was fromAlfa Aesar; selenium (99.99% purity) and dicyclohexylphosphine (98%purity) were from Strem; diphenylphosphine (DPP, >90% purity, 99.9% bycertificate of analysis) and hydroquinone (+99% purity) were obtainedfrom Aldrich; and anhydrous cadmium acetate (99% purity) was fromProchem. Tetradecylphosphonic acid (TDPA, 98% purity) was eitherobtained from Alfa or synthesized using methods well known in the art(Kosolapoff, et al., J. Am. Chem. Soc. 67:1180-1182 (1945). All reagentswere used as received without further purification.

Example 1 Reactions with Phosphines Added

TOPO (6.0 g) and TDPA (0.577 g) were combined in a three-neck roundbottom flask equipped with a stir bar, a thermocouple attached to atemperature controller unit, and a condenser connected to anitrogen/vacuum manifold. The third neck was sealed with a septum. Theatmosphere inside the reactor was evacuated once and refilled with drynitrogen. Inside an inert atmosphere glove box, a cadmium precursorsolution (0.5 m) was prepared by combining cadmium acetate (21.6 g) andTOP (166 g) and allowing the mixture to stir for ˜24 hours until fullydissolved. An aliquot (2.03 g) of this solution was diluted with TOP(3.6 mL) and injected via syringe into the reaction vessel. A 16-gaugeneedle was inserted into the septum of the reaction vessel so that thevessel could be continuously flushed with nitrogen for approximately 10min as the reaction was heated to 250° C. The needle was removed andheating continued to 260° C. Heating was continued at 260-270° C. for 10min. The reaction was cooled to 100° C. and vacuum degassed for 30 minbefore the vessel was returned to a nitrogen atmosphere. An amount ofphosphine (table below) was added as one portion via syringe. Theselenium stock solution was prepared by combining selenium (3.2 g) andTOP (66.0 g) inside the inert atmosphere glove box. The temperaturecontroller attached to the reaction vessel was set to 290° C. and at the270° C. mark, selenium stock (1.4 mL) was rapidly injected to inducenanoparticle formation. Small aliquots were removed periodically fromthe stirring reaction and diluted in hexane so that emission spectracould be obtained as a function of reaction time. TABLE 1 AdditiveProportion Amount Dicyclohexylphosphine 1× 415 μL Diphenylphosphine  0.25× 90 μL Diphenylphosphine 1× 360 μL Diphenylphosphine 5× 1.8 mLDiphenylphosphine 10×  3.6 mL

Example 2 Reactions with Hydroquinone Added

These reactions were carried out as described in Example 1, with thefollowing modifications. No phosphine-based reaction promoter was addedto these reactions. Instead hydroquinone (0.226 g) was combined with theTOPO and TDPA solids prior to the nitrogen flush of the reactor in thefirst step.

Example 3 Reactions Demonstrating the Use of Air as a Reagent

In this example, TOP was obtained from Fluka and used as received. Asolution of Se was prepared by dissolving Se (3.16 g) in TOP (33.2 g)TOPSe). Separately, a cadmium precursor stock solution was prepared bydissolving anhydrous cadmium acetate (6.15 g) in TOP to a final volumeof 40 mL (cadmium stock solution). In each of three round bottom flasks,TOPO (5.0 g) was combined with cadmium stock solution (1.4 mL), TDPA(0.52 g), and TOP (1.1 mL) and heated to 250° C. while continuouslyflushing the vessel with N₂. Once the temperature reached 250° C., thenitrogen flush was halted and the temperature was increased to 270° C.This temperature was maintained for 20 min and the solutions were cooledto 100° C. Using a large-bore needle, dry air was directed into each oftwo flasks at a rate of 200 ml/min for a duration of 1 minute of 10minutes. A third flask received dry nitrogen for 10 minutes at the sameflow rate. Stirring of the solution was maintained throughout. After theexposure period, the flasks were evacuated and refilled with drynitrogen. This was repeated once. The flasks were then reheated to 270°C. and an aliquot of the previously prepared TOPSe solution (1.4 mL) wasrapidly injected. The reaction temperature was maintained at 270° C.while small samples were periodically removed. Reactions were stopped bycooling to 100° C.

The time period between injection of TOPSe and the first appearance ofcolor was noted. This “induction time” is related to the reactivity ofthe solution. Yields of the reactions were determined by the peakband-edge absorbance, normalized for particle size. Results arepresented in Table 2 and FIG. 6. TABLE 2 Condition, time Induction time,Relative particle Peak absorbance of exposure to air seconds yieldwavelength, nm 10 minutes dry 45 1 582 nitrogen 1 minute dry air 6 2.8561 10 minutes dry air 2 18.7 510

This data indicates that exposure to air resulted in a higher yield thanthe control conditions, and that the yield is modulated by the length ofthe air exposure period. Further, it can be seen that the time courseevolution of particle size and particle size distribution, is alsocontrollable by the method of the invention. This shows that thereaction can be tuned to achieve a target size while still maintaining ahigh yield and good particle size uniformity.

Example 4

Cadmium acetate/TOP stock and TOPSe were prepared as in Example 3. In around bottom flask, 2.5 g TOPO was combined with 0.703 mL cadmiumacetate/TOP stock, 0.26 g tetradecylphosphonic acid and 0.55 mL TOP andheated to 250° C. while sparging with N₂. Once the temperature reached250° C., sparging was stopped, the temperature was increased to 270° C.and held at this temperature for 20 minutes. The solution was cooled to100° C. One neck of the flask was opened and dry compressed air wasdirected into the stirring solution for 10 minutes. After the exposureperiod, the flask was evacuated and refilled with dry nitrogen. This wasrepeated two more times. The flask was then reheated to 240° C. and 0.7mL of TOPSe was rapidly injected. After 15 seconds, the reaction wasstopped by injecting 5 mL TOP and removing the heat source. An aliquotwas removed and measurement showed the band-edge absorbance peak at 448nm with the luminescence peak at 471 nm with 31 nm FWHM.

This data illustrates the ability to independently control thefundamental aspects of the crystal growth process, thereby enabling thehigh yield synthesis of very small CdSe nanoparticles.

Example 5 Reactions Making Use of Pre-Air-Treated Reagents

In this example, TOP was obtained from Fluka and used as received. Thecadmium precursor and TOPSe were prepared as in Example 3. In each oftwo round bottom flasks, TOPO (3.0 g) was combined with cadmium stock(0.76 mL), TDPA (0.282 g), and TOP (1.24 mL) and heated to 250° C. whilecontinuously flushing with dry nitrogen. Once the temperature reached250° C., nitrogen-flushing was halted, and the temperature was increasedto 270° C. and held at this temperature. In a separatenitrogen-blanketed flask, TOP (˜5 mL) was heated to 100° C. Once thetemperature reached 100° C., the flask was opened to air and held at100° C., while stirring, for 50 minutes. The flask was then closed andevacuated, followed by refilling with nitrogen. This purge/refill wasrepeated one more time. To one flask containing the cadmium solution (at270° C.) was added air-exposed TOP (1 mL). To the othercadmium-containing flask (at 270° C.) was added unexposed TOP (1 mL).Using a syringe, an aliquot of TOPSe stock (0.71 mL) was rapidlyinjected into each flask. The temperature was maintained at 270° C.while small samples were periodically removed. Reactions were stopped bycooling to 100° C.

The time period between injection of TOPSe and the first appearance ofcolor was noted. Yield of the reactions was determined by the peakband-edge absorbance, normalized for particle size. Results arepresented in Table 3 and FIG. 7.

This data indicates that exposure of TOP to air generates a materialthat can be added to a reaction mix, resulting in higher yield and withuseful modulation of particle size and particle size distribution. TABLE3 Exposure, time Induction time, Relative particle Peak absorbance ofTOP to air seconds yield wavelength, nm None 18 1 583 50 minutes 4 2.875579

All patents, publications, and other published documents mentioned orreferred to herein are incorporated by reference in their entireties.

It is to be understood that while the invention has been described inconjunction with the preferred specific embodiments thereof, that theforegoing description as well as the examples that follow, are intendedto illustrate and not limit the scope of the invention. It should beunderstood by those skilled in the art that various changes may be madeand equivalents may be substituted without departing from the scope ofthe invention, and further that other aspects, advantages andmodifications will be apparent to those skilled in the art to which theinvention pertains.

1-27. (canceled)
 28. A method of producing nanoparticle shellscomprising: (a) mixing nanoparticles with at least one coordinatingsolvent to form a first mixture; (b) heating the first mixture to atemperature that is sufficiently high to form a shell on thenanoparticles when first and second precursors are added; (c)introducing first and second precursors into the first mixture to form asecond mixture thereby resulting in the formation of shells on aplurality of nanoparticles; and (d) cooling the second mixture to stopfurther growth of the shell; wherein the method further comprisesexposing the first or second mixture to a reaction promoter selectedfrom the group consisting of oxygen and a reducing agent.
 29. The methodof claim 28 wherein the reaction promoter is a reducing agent.
 30. Themethod of claim 29 wherein the exposing step comprises adding thereducing agent to the mixture.
 31. The method of claim 29 wherein thereducing agent is a chemical reducing agent selected from the groupconsisting of tertiary, secondary, and primary phosphines; amines;hydrazines; hydroxyphenyl compounds; hydrogen; hydrides; metals;boranes; aldehydes; alcohols; thiols; reducing halides; andpolyfunctional reductants.
 32. The method of claim 29 wherein thereducing agent is a cathode.
 33. The method of claim 32 wherein thecathode is made of a material selected from the group consisting ofplatinum, silver and carbon.
 34. The method of claim 28 wherein thereaction promoter is oxygen.
 35. The method of claim 34 wherein theexposing step comprises directly exposing the mixture to a source ofoxygen.
 36. The method of claim 34 wherein the oxygen is formed in situ.37. The method of claim 36 wherein the oxygen is formed by a redoxreaction.
 38. The method of claim 37 wherein the exposing step comprisesadding a reducing agent selected from the group consisting of tertiary,secondary, and primary phosphines; amines; hydrazines; hydroxyphenylcompounds; hydrogen; hydrides; metals; boranes; aldehydes; alcohols;thiols; reducing halides; and polyfunctional reductants.
 39. The methodof claim 34 wherein the exposing step comprises exposing a coordinatingsolvent to a source of oxygen and adding the exposed coordinatingsolvent to the mixture.
 40. The method of claim 28 wherein thenanoparticles are semiconductive.
 41. The method of claim 28 wherein anadditive or additive precursor is included in step (a). 42-54.(canceled)